Method and a composite for mercury capture from fluid streams

ABSTRACT

A method for removing mercury from a fluid stream includes the steps of providing a porous composite material comprising a substrate and a plurality of catalyst and/or photocatalyst particles, and contacting substrate with a fluid stream. The porous composite material adsorbs and/or then oxidizes or reduces metallic species including elemental mercury. A fossil fuel fired power plant can include an emission control device comprising the porous composite material to filter flue gas emissions into the environment.

This application claims benefit under 35 U.S.C. § 119(e) to U.S.Provisional Application Ser. No. 60/452,572, filed Mar. 6, 2003, thedisclosure of which is incorporated herein by reference.

STATEMENT REGARDING FEDERALLY SPONSORED RESEARCH OR DEVELOPMENT

The U.S. Government has certain rights to the invention based onEnvironmental Protection Agency Grant/Contract No. R-82960201 andNational Aeronautics and Space Administration Grant/Contract No. NCC9-110, both with the University of Florida.

BACKGROUND OF THE INVENTION

1. Field of the Invention

This invention is directed to methods and composite materials forpurifying fluid streams. For example, fields of the invention includeflue gases emitted from combustion sources (e.g., coal-fired powerplants, waste-to-energy facilities, medicinal and similar incinerators,and industrial manufactures) and for purifying ground, surface, and/orindustrially processed waters. More particularly, the invention relatesto removal of mercury and other contaminants from a fluid stream byadsorption and either subsequent or continuous catalytic andphotocatalytic oxidation using catalyst and photocatalyst impregnated ordoped sorbents (e.g., silica-gels).

2. Description of the Related Prior Art

Mercury emission from combustion sources is a significant environmentalconcern. When mercury is released into the atmosphere, it can betransported by wind and then through direct deposition can accumulate insurface waters. In the water, biological processes can transform mercury(typically elemental mercury) into methylmercury, which is highly toxicand can bioaccumulate in fish. Therefore, preventing the release ofmercury into the environment is very important.

The largest emitters of mercury are coal-fired electric utility plants,which account for an estimated more than 90% of all anthropogenicmercury emissions. Mercury is also listed as one of the 189 hazardousair pollutants (HAPs) in the 1990 Clean Air Act Amendments (CAAA).Regulations are being set for future emission standards for combustionsources to be implemented as early as 2007.

The unique feature of mercury emission that differs from other toxicmetals results from mercury's 5d¹⁰6s² closed shell electronic structurethat is isoelectronic to He (1 s²), and is accordingly highly stable inits elemental state. As a result, unlike other toxic metals, thedominant form of mercury in combustion exhaust is elemental mercury(Hg⁰) vapor, unless chlorine is present. Since Hg⁰ is insoluble, gasremoval devices, such as scrubbers, are also ineffective for itsremoval. Similarly, particulate removal devices (e.g., baghouses andelectrostatic precipitators) are also highly ineffective.

In recent years, numerous studies for enhanced mercury removal fromcombustion sources have been undertaken. Generally the methods used inthese studies were based on either adsorption or oxidation. Currently,the maximum available control technology (MACT) for mercury is powderedactivated carbon injection. However, its predicted use is limitedbecause of questionable costs, presently low capacity, low applicabletemperature range, and problems associated with collection andregeneration of the carbon. This approach has been estimated to costabout $2-5 billion annually to implement in U.S. coal-fired powerplants.

In addition to activated carbon, calcium-based sorbents such as hydratedlime have also been considered. However, calcium-based sorbents providepoor efficiency for mercury removal unless they are modified with flyash. Studies have also been carried out using zeolite and bentonite, butthey have demonstrated very low capacity for mercury.

The use of an advanced oxidation process has also been investigated formercury removal. Heterogeneous photocatalysis is one such method thatutilizes a semiconductor in the oxidation and mineralization ofpollutants, either in the air phase or water phase. When the surface ofthe photocatalyst absorbs a specific amount of energy (usually aphoton), an electron from the valence band is promoted to the conductionband, thereby leaving a positive charged “hole” in the valence band.Reactions with these electron-hole pairs result in the formation ofhydroxyl radicals (OH⁻), which are very reactive oxidizing species.Titanium dioxide (TiO₂) is one such semiconductor/catalyst that can beactivated when irradiated by UV light.

Wu et al. (Env. Eng. Sci. 1998;15(2):137-148) and Lee et al. (AlChE J2001;47:954-961) used TiO₂ nano-aerosols generated in-situ in combustionsystems to effectively transform elemental mercury into mercuric oxide.This process was reported to have high efficiency, but a majorlimitation related to separation and regeneration of the mercuricoxide-loaded TiO₂ aerosols.

Therefore, an economical solution that can more efficiently capturemercury compared to the technologies discussed above offers thepotential to significantly decrease the estimated costs to meet pendingregulations. In addition, the solution should be engineered such that itcould be easily implemented in existing coal-fired power plants andsimilar installations. Not to be bound by theory, but a viable locationfor the technology derived herein would be to insert the technologybetween the electrostatic precipitator or baghouse and the effluentstack for coal-fired power plants, although the technology is notlimited to this location. In addition, it is conceivable to micronize(create a fine powder with diameters less than 45 μm) and inject thematerial in to a flue duct.

SUMMARY OF THE INVENTION

In accordance with the present invention, there is provided a method andcomposite for removing mercury from a fluid stream, the method includingthe steps of contacting a composite material comprising a substrate andcatalyst particles with a fluid stream. The composite material adsorbsand oxidizes the mercury.

Preferably, the catalyst particles are located on the substrate surfaceand/or contained in the substrate. The composite material may be asorbent and, if so, is preferably a gel, more preferably, a xerogel.

The method of the invention preferably includes the step of irradiatingthe composite material with radiation, preferably radiation having awavelength of from about 160 to about 680 nm. The substrate ispreferably transparent to radiation and, for example, may be poroussilica, and the catalyst may comprise TiO₂. Preferably, the sorbent is amaterial having a surface area (BET) of about 1 to about 1500 m²/g,preferably about 200 to about 900 m²/g. The catalyst is preferablypresent in the composite material in an amount of from about 0.1 wt % toabout 100 wt %.

The method of the present invention also preferably comprises the stepof regenerating the composite. The regeneration may be either chemicalor thermal regeneration.

The present invention also relates to a composite, the compositeincluding a sorbent and mercuric oxide and preferably further includinga catalyst. If a catalyst is present, it may be present in an amount ofabout 0.1 wt % to about 100 wt %. The catalyst is preferably aphotocatalyst, more preferably, TiO₂. The sorbent is preferably a gel,more preferably, a xerogel. The sorbent is preferably silica, andpreferably has a surface area (BET) of from about 1 to about 1500 m²/g,preferably about 200 to about 900 m²/g. The composite preferablycontains the mercuric oxide in an amount of from about 0.1 wt % to about100 wt %, more preferably about 0.1 wt % to about 10 wt %.

BRIEF DESCRIPTION OF THE DRAWINGS

A fuller understanding of the present invention and the features andbenefits thereof will be accomplished upon review of the followingdetailed description together with the accompanying drawings, in which:

FIG. 1 (a)-(c) illustrate scanning electron microscope (SEM) and energydispersive spectrometry (ADS) analyses of crushed composite pelletsincluding a SEM image, Si mapping and Ti mapping, respectively.

FIG. 2 illustrates a schematic of an exemplary photocatalytic adsorptionpacked bed reactor based system for mercury vapor removal equipped witha source of mercury vapor and an analyzer for measuring mercuryconcentrations.

FIG. 3 illustrates dimensionless outlet mercury concentration (C/Co) asa function of time (13 wt % TiO₂ loading, relative humidity of 70% andresidence time of 0.29 sec).

FIG. 4 illustrates mercury removal efficiencies for various TiO₂loadings (15% relative humidity, 0.35 sec residence time).

FIG. 5 illustrates mercury removal efficiencies for various residencetimes (13 wt % TiO₂, 70% relative humidity).

FIG. 6 illustrates mercury removal by adsorption alone and then removalby adsorption and irradiation (10% relative humidity, 12 wt % TiO₂loading)

FIG. 7 illustrates mercury removal via simultaneous adsorption and UVirradiation (70% relative humidity, 12 wt % TiO₂ loading)

DETAILED DESCRIPTION OF THE INVENTION

The invention includes a new method and composite that can removemercury from a fluid stream. Specifically, the invention is targeted toremove mercury via adsorption and/or either simultaneous or subsequentoxidation. Adsorption on the composite material allows mercury to beconcentrated while exposure to radiation ensures the oxidation of theadsorbate(s). Intermittent UV light exposure can be used with theinvention which minimizes energy consumption of the process if sodesired. High efficiency, large capacity and the ability to recovermercury are advantageous features of the invention.

As used herein, the term “mercury” refers to all forms of mercuryincluding oxidized states (e.g., HgO, HgCl, HgCl₂) and elemental mercury(Hg⁰). As used herein, the term, “impregnated” refers to theincorporation of a material (e.g., a photocatalyst) within the porousnetwork of a sorbent, and may be either attached to the surface of thepores and/or a part of the crystalline network. Also, as used herein,the term “doping” refers to the addition of a material such that it isfixed to the sorbent internal or external surface and is accessible tothe fluid stream. Further, as used herein, the term “sorbent” refers toan amorphous or crystalline solid that is capable of accumulatingcontaminants on or within the porous network of the sorbent.

The porous composite material preferably consists of a high surface areasubstrate material. For example, a silica-gel impregnated withphotocatalyst particles, such as TiO₂; herein referred as a “SiO₂—TiO₂composite gel”. The SiO₂—TiO₂ composite gel can provide a surface areaof about a few m²/g to 1500 m²/g. The gel is preferably a xerogel,defined as a gel that is obtained by evaporation of the liquid componentat ambient pressure and temperatures below the critical temperature ofthe liquid. However, other gel forms may be used with the invention.Other suitable substrates include activated carbon, ceramics, metalsilicates, alumina, zeolites, and the like as well as nonporoussubstrates such as silica/glass beads, stainless steel, and the like.

The mercury deposited on the composite following irradiation has beenidentified as mercuric oxide. Very high Hg removal efficiency (at leastabout 99%) can be achieved with continuous irradiation. It has also beenobserved that photocatalytic oxidation “activates” the adsorbent, thusenhancing the subsequent adsorption capacity of the composite materialwhen UV irradiation is not applied.

The capacity of the porous composite material can be further increasedby optimizing the mass transfer of mercury from the bulk fluid phase tothe adsorption sites. For example, one could manipulate the gels poresize distribution or decrease the pellet size from the current 5×3 mmsize tested herein. If desired, by rinsing the composite pellets with asuitable acid, such as H₂SO₄ and/or HNO₃, adsorbed mercury can beseparated from the pellets that permit the adsorption sites on thecomposite to be regenerated. More efficient regeneration might beobtained by thermal treatment of the mercury at elevated temperaturessuch as about 200° F. to about 1000° F.

Photocatalyst particle (e.g. TiO₂) loading at all levels has been foundto enhance mercury removal. Optimal loading is a function of sorbentporosity, surface area, transparency to UV light, permeability,adsorption characteristics, granular size, and other physical andchemical characteristics. In the preferred embodiment a TiO2 loading ofbetween 10 and 13 wt % has been shown to give optimum performance.

In a preferred embodiment, the SiO₂—TiO₂ composite gel is formed using asol-gel method. However, other methods to form the composite will beapparent to those skilled in the art. The basic formula uses specificvolumetric ratios of various acids, water, silica alkoxide (silicaprecursor) or sodium silicate, with or without, various cosolvents.During formulation, during gelation, or post gelation the silica isdoped, for example, with a commercially available photocatalyst, such astitanium dioxide. Preferably, the titania percentage varies from about0.5% to about 15% on a wt/wt basis, but TiO₂ loadings up to 100 wt % canbe incorporated. Mixed alkoxide synthesis can also be used to form acomposite gel of SiO₂ and TiO₂ with a more homogeneous distribution ofTiO₂. Various synthesis and aging steps can produce composites with poresizes ranging from <10 angstroms to >50 nm or as large as desired.Preferably, the pore sizes are greater than about 30 angstroms and lessthan about 320 angstroms, more preferably between about 60 and 200angstroms, and most preferably between about 100 and 140 angstroms. Inaddition, surface treatments can be used to enhance Hg adsorption. Whenthe solution becomes viscous during the gelation step, it may then betransferred into a mold in order to create a pellet of a desired size.After gelation, the composite may then be aged for varying lengths oftime to increase its strength. After aging, the pellets may then beremoved from their mold, rinsed with water, and then placed in anothercontainer for additional heat treatments. In the preferred embodiment,the pellets are placed in an oven and the temperature may then be rampedfrom room temperature to 103° C. and kept constant for 18 hours,resulting in vaporization of the liquid within the porous silica matrixto form a xerogel. The temperature may then be ramped to 180° C. andkept constant for 6 hours. Additional curing at higher temperatures canalso be achieved (up to 600° C.) for strengthening of the gel. Theresultant average pore size of the gel can range from a pore size ofabout 30 angstroms to a pore size of between about 100 to about 200angstroms, depending on the initial formula. The pellets can then beused in a packed-column.

This indicates only one exemplary composite formulation. A wide varietyof formulations, catalysts, aging and drying parameters can be used toderive the optimum pore size, pellet/particle size, surface area,surface adsorption characteristics, reduction efficiency, permeability,temperature stability and regeneration characteristics. Alternatively,the sorbent can be synthesized in bulk and crushed or ground andscreened to produce granular particles of the optimum size range forvarious applications.

A significant difference between the composites described herein andother composites for mercury removal is the use of a UV transparentsubstrate material such as silica. Porous silica is a good adsorbentmedium that is also substantially optically transparent to UV light,which allows the penetration of UV light through its matrix to activatethe intermixed photocatalyst particles, such as titanium dioxide.Preferably, the photocatalyst particles are provided both on the surfaceof and within the silica matrix allowing oxidation to occur on bothexternal and internal surfaces within the porous silica structure.

A wide variety of photocatalysts can be used with the invention. Thesol-gel process is not limited to the use of titanium dioxide, but othercatalysts such as HgO, ZnO, V₂O, SnO₂ or even modified TiO₂ catalystscoated with platinum or other conductive materials can also be used. Inaddition, the composites can be made into any shape convenient for use,such as spheres, cylinders, or other shapes.

EXAMPLES

The present invention is further illustrated by the following exampleswhich include demonstrations of the superior performance of the advancedporous composite material for elemental mercury removal. The examplesare provided for illustration only and are not to be construed aslimiting the scope or content of the invention in any way.

Example 1 Synthesis of Silica-Titania Composite

The silica-titania composites were made by a sol-gel method using nitricacid and hydrofluoric acid as catalysts to increase the hydrolysis andcondensation rates, thereby decreasing the gelation time. The basicformula used to create gels with a pore size of roughly 150 Å is asfollows: 25 mL water, 50 mL ethanol, 35 mL TEOS(tetraethylorthosilicate), 4 mL nitric acid (1N), and 4 mL HF (3%). Ofcourse, one of ordinary skill in the art will recognize that siliconalkoxides, sodium silicate, colloidal silicas, slip casting ortraditional ceramic techniques are suitable for use with the invention.

The chemicals were reagent grade and were added individually, in noparticular order, to a polymethylpentene container. During this time, aknown mass of Degussa (Dusseldorf, Germany) P25 TiO₂ was added to thebatch and the percentage of titania recorded is given as a percent byweight of silica. A magnetic stir plate provided sufficient mixing, butcare should be used to insure that the TiO₂ is well dispersed in the soland that the homogeneous distribution of TiO₂ is maintained throughoutthe gelation process. The solution (including the P25) was pipeted intopolystyrene 96-well assay plates before complete gelation. The volumeadded to each well was approximately 0.3 ml. After gelation, the plateswere then covered with lids and wrapped in foil to prevent prematureevaporation. Next, the sample was aged at room temperature for two days,then at 65° C. for two days.

After aging, the pellets were removed from the container, rinsed withdeionized water to remove any residual acid or ethanol, and placed in aTeflon container for the next series of heat treatments. A small hole inthe lid of the container allowed slow and uniform drying of the gel. Thepellets were then placed in an oven and the temperature was ramped fromroom temperature to 103° C. (2°/min) and kept constant for 18 hours,resulting in the vaporization of liquid solution within the silicanetwork. Next, the temperature was ramped to 180° C. (2°/min) forremoval of physically adsorbed water and hardening of the gel, where itwas kept constant for 6 hours and then was slowly decreased back to roomtemperature over a 90 minute period. The resultant size of an individualcylindrical pellet after drying was approximately 5 mm in length with adiameter of 3 mm.

The BET (Brunauer, Emmett, and Teller equation) surface area and porevolume analyses were performed on a Quantachrome NOVA 1200 Gas SorptionAnalyzer (Boynton Beach, Fla.). The samples were outgassed at 110° C.for approximately 24 hours and analyzed using nitrogen adsorption. Theaverage pore size was calculated from the total pore volume and thesurface area. Pore size distribution curves were also attained toprovide additional information on pore morphology. Scanning ElectronMicroscopy (SEM) (JSM-6400, JEOL USA, Inc.) with Energy DispersiveSpectroscopy (EDS) detector (Tracor System II, Oxford Instruments, Inc.)was used for morphology and surface elemental analysis. Thesilica-titania gel composites had specific surface areas on the order of200 to 300 m²/g, pore volumes around 1 cc/g, and average pore diametersof about 150 angstroms. The synthesized pellets had a white color due tothe presence of TiO₂. The addition of TiO₂ in the range studied did notseem to significantly affect the surface area. The pore volume withinthe loading range was roughly 1.0 cc/g and had negligible differencesamong the various pellets. Concerning pore size, the average porediameter (pore volume/surface area) averaged 150 angstroms. The SEMimage of crushed fresh pellet (13 wt % loading) is shown in FIG. 1(a).The corresponding EDS elemental mappings of Si and Ti are shown in FIG.1(b) and FIG. 1(c), respectively. FIG. 1(c) shows TiO₂ was welldistributed in the SiO₂ matrix although some agglomerated TiO₂ can alsobe seen.

Example 2 Mercury Removal Characterization/Methodology

Silica-Titania Composite Gel formed using the synthesis method describedabove was tested in a packed bed reactor system to characterize themechanisms and efficiency for mercury vapor removal. The reactor systemincluding a supply of mercury vapor and a Hg analyzer is shown in FIG.2. The flow-rate of mercury-laden air was 0.67 liters/min with aresidence time in the reactor of 0.29 seconds. The initial mercuryconcentration for experiments ranged from 7 to 150 ppb. Mercury vaporladen air was introduced into the system by passing purified air aboveliquid mercury held in a reservoir. To study the effects of moisture onthe system, water vapor was introduced by bubbling water using purifiedair. The mercury concentration of the mixture was measured by a UVmercury analyzer (VM 3000, Mercury Instruments or Zeeman RA-915 mercuryanalyzer). The air carrying the designated level of mercuryconcentration and humidity flowed downward through the packed-bedreactor from the top in order to minimize the chance of selective flowor channeling through the reactor.

A stainless steel mesh (64 um opening) was used to hold the pellets. AUV lamp (4W) was placed at the center of the paclced-bed reactor, andthe pellets were randomly packed around the lamp. Between 5 and 10 gramsof pellets were used in the experiments. The cross-sectional area of thereactor was 26.5 cm². After flowing through the reactor, dilution airwas introduced to dilute the mercury concentration to the appropriaterange for measurement. The air was then passed through a carbon trapbefore it was exhausted into a hood while a slit of the air was directedto the mercury analyzer for measurement. Purge air was used to flushmercury out of the system after each experiment.

After the experiment, the composite pellet was analyzed by BET again toexamine if there was any significant change in surface area. The amountof total mercury adsorbed on pellets was determined following a hot aciddigestion (HNO₃:H₂SO₄ mixture; 7:3) of 25 mg of pellets/10 ml ofsolution. Samples were brought to a refluxing boil on a hot plate for 4hours. After cooling, 0.1 ml of concentrated HCl was added to thesamples, and the final volume adjusted to 50 ml by dilution withNanopure® water. Mercury concentration was then measured by InductivelyCoupled Plasma spectroscopy (ICP) to determine the capacity.

Example 3 Mercury Removal Rate as a Function of Adsorption and Oxidation

FIG. 3 shows the dimensionless outlet mercury concentration (C/Co) forthe UV on/off cycles as a function of time. The inlet concentration wasregularly checked to ensure it stayed at the designated level. As shown,the outlet concentration in the first cycle increased to 68% after 15minutes, proving that the high surface area silica gel was capable ofadsorbing mercury. The breakthrough time was short due to the small bedheight used, but it can be easily made larger by using a longer bed.

After 15 minutes of adsorption, the UV-light was turned on. The outletconcentration quickly dropped down to 0% in less than 2 minutes,demonstrating highly effective photocatalytic oxidation. During thisoxidation period, effluent mercury stayed at this low level. After 6minutes of UV exposure, the UV light was turned off to start the secondcycle. The outlet concentration remained at a low concentration for ashort period of time and then increased in a similar pattern observed inthe previous cycle. Comparison of the end of the first cycle before theUV light was turned on and the beginning of the second cycle indicatesthat photocatalysis oxidized the previously adsorbed mercury and“reactivated” the silica gel. Otherwise, the mercury concentrationinitially measured in the second cycle would be the final level detectedfrom the previous cycle (i.e. 68%).

The other unexpected phenomenon demonstrated was that oxidation cyclesimproved adsorption for the next cycle. In other words, by comparing themercury outlet concentrations of the respective cycles, a decreasingtrend was observed.

In a related experiment, the time to reach 20% exhaustion (i.e., 20% ofthe sorbent's capacity utilized) for each cycle for 10 grams of pelletswas measured. As can be seen from Table 1, the pellets performed betterwith each successive cycle. TABLE 1 Time for Reaching 20% Exhaustion byAdsorption in the Various Cycles Cycle No. 1 2 3 4 5 6 Time (min) 1 2.13.2 6 7.2 9.3

Thus, Table 1 clearly shows the increase in time with each cycle toreach 20% exhaustion (e.g. 1 min in the first cycle and 2.1 minutes inthe second cycle). This breakthrough profile became stable after a fewcycles.

Example 4 TiO₂ Loading

An increase in TiO₂ loading to a certain extent is expected to yieldhigher mercury removal efficiencies by providing more active sites forphotocatalytic oxidation. However, higher loadings (i.e., greater wt %ratio) may interfere with the adsorption or hinder UV transparency,therefore reducing the effectiveness. Measured efficiencies for variousTiO₂ loadings are shown in FIG. 4. The efficiencies of photocatalyticoxidation, adsorption at 5 minutes and at 15 minutes are both reported.

When the UV light was on, mercury removal was 100%. In looking atadsorption, the 2.8% impregnated silica-gel clearly had a lower capacitythan the other two loadings. This deficiency may be due to thephotocatalytic “activation” discussed in the previous section. The 2.8%TiO₂ loading may not be enough to provide the necessary OH radicals foractivation, thus resulting in a lower adsorption capacity. Comparing the13% and 18% data, the 13% provided a slightly better performance but thedifference did not appear to be significant.

For optimum system performance, TiO₂ particles should be dispersed.Agglomeration appears to yield less effective use of TiO₂ for thispurpose. The experimental results suggest that 13% loading to be theoptimal based on the current doping methodology.

Example 5 Residence Time

Flow rate is another important operating parameter that generallydetermines the mercury removal efficiency in the system. The flow ratecontrols the residence time of the mercury containing gas in the reactorand therefore the effectiveness of adsorption and reaction can meimpacted. In addition, by varying the flow rate, the rate limitingmechanism can be identified. The removal efficiency as a function ofresidence time is shown in FIG. 5.

As the residence time decreased from 0.78 to 0.16 s, adsorption wasgreatly impacted. The removal efficiency drastically decreased when theresidence time decreased. Compared to adsorption, the removal efficiencyby photocatalytic oxidation only decreased slightly, although it wasmuch more affected at the smallest residence time. While short residencetime reduces the performance of the system regardless of whetheradsorption or oxidation is the main removal mechanism, the resultsclearly indicate that adsorption is the rate limiting factor.

Example 6 Continuous UV Versus Cyclic Operation

An alternative to exposing the silica-gels to intermittent UV is tomaintain an environment of constant irradiation. FIG. 6 demonstratesthat once UV was applied to the system, the effluent mercuryconcentration returned to zero and remained there for the duration ofthe experiment. Similarly, FIG. 7 demonstrates that if the system isirradiated from the beginning, other than the fluctuation in effluentmercury concentration in the beginning of the experiment, the effluentconcentration remained at zero for the duration of the experiment.Similar experiments were carried out for almost 500 hours with the sameresults. Furthermore, silica impregnated with HgO with and without TiO₂performed similarly in the presence and absence of UV light.

It is to be understood that while the invention has been described inconjunction with the preferred specific embodiments thereof, that theforegoing description as well as the examples which follow are intendedto illustrate and not limit the scope of the invention. Other aspects,advantages and modifications within the scope of the invention will beapparent to those skilled in the art to which the invention pertains.

1. A method for removing mercury from a fluid stream, comprising thesteps of: providing a composite material comprising a substrate andcatalyst particles; and contacting a fluid stream with said composite,wherein said composite adsorbs and/or oxidizes said mercury.
 2. Themethod of claim 1, wherein said catalyst particles are on the substratesurface and/or contained in the substrate.
 3. The method of claim 1,wherein said composite material is a sorbent.
 4. The method of claim 3,wherein said sorbent is a gel.
 5. The method of claim 4, wherein saidgel is a xerogel.
 6. The method of claim 1, further comprising the stepof irradiating said composite material with radiation.
 7. The method ofclaim 6, wherein said radiation has a wavelength of from about 160 toabout 680 nm.
 8. The method of claim 1, wherein said substrate istransparent to radiation.
 9. The method of claim 8, wherein saidsubstrate comprises porous silica.
 10. The method of claim 9, whereinsaid catalyst comprises TiO₂.
 11. The method of claim 3, wherein saidsorbent has a surface area (BEI) of about 1 to about 1500 m²/g.
 12. Themethod of claim 1, wherein said catalyst is present in said compositematerial in an amount of from about 0.1 to about 100 wt %.
 13. Themethod of claim 1, further comprising the step of regenerating thecomposite.
 14. The method of claim 13, wherein said regeneration stepcomprises chemical or thermal regeneration.
 15. A composite, comprisinga sorbent and mercuric oxide.
 16. The composite of claim 15, furthercomprising a catalyst.
 17. The composite of claim 16, wherein saidcatalyst is present in said composite in an amount of about 0.1 to about100 wt %.
 18. The composite of claim 16, wherein said catalyst is aphotocatalyst.
 19. The composite of claim 18, wherein said photocatalystis TiO₂.
 20. The composite of claim 15, wherein said sorbent is a gel.21. The composite of claim 20, wherein said gel is a xerogel.
 22. Thecomposite of claim 15, wherein said sorbent is silica.
 23. The compositeof claim 15, wherein said sorbent has a surface area (BET) of from about1 to about 1500 m²/g.
 24. The composite of claim 15, wherein saidmercuric oxide is present in said composite in an amount of from about0.1 to about 100 wt %.